The present invention relates to a silicone resin-mica laminate and a method of manufacturing thereof. More particularly, the invention relates to a silicone resin-mica laminate which has excellent water resistance, electrical insulation and heat resistance, and whose properties are stable even when in contact with water, and to a method for its manufacture.
Mica is known to be a material having extremely good electrical insulation and heat resistance, and silicone resin on the other hand is known to have excellent electrical insulating and heat resistance properties.
Lately, a silicone resin-mica laminate has come to be used in the form of a laminate of mica films bound together by a silicone resin binder. This kind of silicone resin-mica laminate takes advantage of the excellent properties of the mica and the silicone resin, such as electrical insulation and heat resistance.
Ordinarily, a silicone resin-mica laminate has good electrical insulation and heat resistance properties. However, when the manufacturing conditions or the conditions under which it is used are unfavorable, the properties of the silicone resin-mica laminate are apt to be ill-affected.
For example, if the silicone resin-mica laminate is maintained for a long period in a place of high humidity, or when it is in contact with moisture, the water penetrates the mica layers, thereby greatly impairing the electrical properties and the mechanical strength.
Various proposals have been made with a view to solving this problem.
Examples are Japanese Laid-open Patent Application Nos. 53-149250, 55-90556, and 53-149251, and Japanese Patent Publication No. 53-777, which disclose inventions relating to a manufacturing method for the production of silicone resin-mica laminate characterized in that a binder solution is prepared by (i) mixing a silicone resin as the binding agent and a phosphoric acid catalyst as the curing catalyst with a silane coupling agent, oxime silane, epoxy resin, etc. and (ii) modifying the catalyst, and in that the mica films are bound together to form a laminate by means of the binder solution.
Although the thus-obtained silicone resin-mica laminates obtained show some improvement in water resistance, the life of the binder solution is so short that the manufacturing condition becomes more restrictive. Also, since certain additives used in the binder solution are only poorly soluble in solvents, the binder solution is difficult to spread uniformly over the surfaces of the laminated mica. Another problem is that, when heated to high temperatures, the silicone resin-mica laminate is susceptible of color change and fuming.
There is also known a method capable of imparting such properties as water resistance to an inorganic substance, which generally comprises the steps of treating the inorganic substance with a silane coupling agent and having a resin body impregnated by the inorganic substance.
Although this method is often effective when applied to conventional organic resins, it fails to sufficiently improve the properties such as water resistance of a laminate if the laminate is a silicone resin-mica laminate made up of mica and silicone resin pre-treated only with silane coupling agent.
Further, the following problems are associated with the manufacturing of a silicone resin-mica laminate.
Generally, a laminated mica has a tendency of adsorbing as well as releasing atmospheric moisture and this moisture affects the quality of the mica so much that it is important to monitor the humidity for proper quality control. When the humidity is high, the silicone resin-mica laminate produced will be poor in mechanical strength and water resistance and on the other hand when the humidity is too low, the resulting silicone resin-mica laminate will be less flexible and very brittle. Therefore, even if pieces of mica from the same lot are used, the resulting silicone resin-mica laminate shows different properties depending on the humidity of the atmosphere at the time when the respective silicone resin-mica laminate is made. For example, it is difficult to maintain the good properties of the silicone resin-mica laminate if it is manufactured during the rainy season when the humidity rises high or during the midwinter when the humidity is very low.
Such effect of humidity is experienced not only seasonally but also in a single day in extremely cases in which the weather of the day changes very drastically. For example, it is possible that a piece of silicone resin-mica laminate manufactured in the morning will have properties much different from those of another piece manufactured in the evening of the same day.
To solve this problem, the plant could be sealed from the outside air or air-conditioned to thereby maintain the indoor humidity at a desirable value. However, these measures are impractical because the large volumes of solvents used in the plant cannot safely be kept therein and sealing or air-conditioning the plant would require a considerable amount of investment for installation of the equipment for those purposes.
As measures for improving the water resisting property of the silicone resin-mica laminate, there have been proposed methods such as disclosed in Japanese Patent Publication No. 50-35240 and Japanese Patent Publication No. 61-10922, the latter being characterized in that an aqueous binder solution containing aminosilane and oximsilane is employed for pre-treatment of the laminated mica, and the former being characterized in that the laminated mica is pre-treated with oximsilane.
The inventors of the present invention first considered whether it would be possible to manufacture a silicone resin-mica laminate scarcely affected by atmospheric moisture by employing those proposed methods. However, their tests indicated that while the water-resistance property of the manufactured silicone resin-mica laminate was certainly improved more or less, the unfavorable effect of the humidity during the manufacturing process was not avoided.
Therefore, there existed no known means effective to improve the water resistance, heat resistance, and electrical insulation properties of silicone resin-mica laminate, and at the same time to render the silicone resin-mica laminate resistant to the effects of humidity.